In the past, carbon material has been used mainly as a negative active material for lithium-ion secondary batteries.
These days, however, in the batteries which utilize carbon material as a negative active material, the discharge capacity is so enhanced as to be close to the theoretical capacity of carbon material. Such a present situation makes it difficult to further improve the discharge capacity of those batteries.
In recent years, therefore, high-capacity negative active materials which can be alternatives to carbon material have been studied thoroughly. As one example of such high-capacity negative active materials, silicon material can be included (refer to Provisional Publication No. 29602 of 1995, for example.)
With silicon being used as a negative active material, however, a battery significantly deteriorates in cycle performance compared to a case of carbon material being used as a negative active material. The reasons for this can be explained as follows: having a large volume expansion associated with absorption of lithium ion, silicon is pulverized easily due to the repetition of charge/discharge; such pulverization creates a portion where a conductive pathway is broken and causes a decrease in current collection efficiency; and, consequently, as the number of charge/discharge cycles grows, the capacity decreases rapidly and the cycle life becomes short.
As described in USP200210086211 and EP1205989A2, Provisional Publication No. 3920 of 1998, and Provisional Publication No. 215887 of 2000, it is proposed that a non-aqueous electrolyte secondary battery utilize the silicon which is coated with carbon material, as a negative active material, in that the carbon material-coated silicon shows better cycle performance than the one without coating.
However, as compared to a conventional lithium ion battery where carbon material is used as a negative active material, the cycle performance of the above proposed battery is still unsatisfactory.
The present invention has been conducted in view of such circumstances. It is an object of the invention to provide a non-aqueous electrolyte battery having a large capacity and a satisfactory cycle life.